The email address that is to be used in the 'From' field
when sending the email using SMTP. The format of this object
is an e-mail address which conforms to RFC822 and RFC1123
'addr-spec' syntax.
Some examples are
raj@helpme.com,
bob@service.com,
mtom@abc.caview.ca.us.
A set operation on this object with a invalid format will
result in an error.